Conventionally, it has been conducted to form a cured coating film of silicone composition on the surface of the substrate such as, paper or a plastic film to impart releasing properties thereto, for the purpose of preventing adhesive substance from adhering or fixing to the substrate.
In this case, examples of the methods for forming a silicone coating film on the surface of the substrate include:
(1) a method in which an organopolysiloxane having aliphatic unsaturated groups is subjected to an addition reaction with an organohydrogenpolysiloxane by using a platinum-group compound as a catalyst to form a release coating film;
(2) a method in which an organopolysiloxane is subjected to a condensation reaction by using an organic acid metal salt catalyst such as an organotin compound to form a release coating film;
(3) a method in which a composition comprising an organopolysiloxane having acryl groups and a photoreaction initiator is irradiated with ultraviolet to form a release coating film; and
(4) a method in which an organopolysiloxane having acryl groups is irradiated with electron beams to form a release coating film. Further, the types are classified into the solvent-type which is a solution in an organic solvent such as toluene; the emulsion-type prepared by emulsifying a silicone; and the solvent-free-type which does not substantially contain organic solvent.
Among those methods for forming a silicone coating film, the addition-reaction-type method (1), which is capable of satisfying various releasability required in low-speed peeling and high-speed peeling, is widely used. In addition, from the standpoints of safety, hygiene and the like, the solvent-free-type composition has been replacing the solvent-type composition recently. Furthermore, a solvent-free-type composition having a lower viscosity is desired because high-speed application is required from the aspect of productivity and from the standpoints of leveling properties and avoiding misting.
Moreover, for the high-gloss labels requiring a higher-grade feeling as well as excellent releasing properties, silicone coating film is required to be formed at a lower temperature and to have satisfactory adhesion to the substrate, from the standpoint of preventing the polyethylene-laminated fine paper used as a substrate from thermal degradation to cause reduced surface smoothness. It is not preferable to increase the amount of an expensive platinum catalyst for improving the low-temperature curability because it results in a cost increase. Furthermore, the composition is desired to be free from silicone migration from the standpoint of preventing the occurrence of printing failures in label surface printing. Simultaneously, impartation of slip properties to the surface of the cured silicone coating film is required for the purpose of preventing the running failures in printers.
As the solvent-free-type composition containing no organic solvent, a composition comprising (a) a diorganopolysiloxane having vinyl groups and having a viscosity of 50 to 4,000 mPa·s at 25° C., (b) an organohydrogenpolysiloxane having at least three silicon-bonded hydrogen atoms per molecule, (c) a platinum catalyst, and (d) an activation inhibitor is disclosed in JP-B-52-47485 (Document 1; the term “JP-B” as used herein means “examined Japanese patent application”). However, this composition not only gives a cured coating film having no slip properties but has poor low-temperature curability. In JP-B-5-23308 (Document 2), a solvent-free-type composition in which the proportion of higher alkenyl groups in the unsaturated groups has been regulated to 50% or more so as to improve low-temperature curability is proposed. However, this composition also gives a cured coating film having no slip properties. On the other hand, as the method for imparting slip properties to a solvent-free-type composition, a method in which a high-molecular organopolysiloxane having a hydroxyl group bonded to the silicon atom located at a molecular chain end is added in addition to an diorganopolysiloxane having vinyl groups and an organohydrogenpolysiloxane is proposed in JP-B-3-19268 (Document 3). However, according to the above method, migration of silicone becomes problematic because the reaction of the silicon-bonded hydroxyl groups in the high-molecular organopolysiloxane with the hydrogen atoms bonded to the remaining silicon atoms in the network is not apt to proceed under low-temperature curing conditions.
As described above, it has been difficult to obtain a solvent-free addition-reaction-type release composition which satisfies all the performance requirements shown above, as achieved by the present invention, without increasing the amount of a platinum catalyst.
Document 1 : JP-B-52-47485
Document 2 : JP-B-5-23308
Document 3 : JP-B-3-19268